By using AMR cosmological hydrodynamic N-body zoom-in simulations , with the RAMSES code , we studied the mass transport processes onto galactic nuclei from high redshift up to z \sim 6 .
Due to the large dynamical range of the simulations we were able to study the mass accretion process on scales from \sim 50 [ kpc ] to \sim few 1 [ pc ] .
We studied the BH growth on to the galactic center in relation with the mass transport processes associated to both the Reynolds stress and the gravitational stress on the disc .
Such methodology allowed us to identify the main mass transport process as a function of the scales of the problem .
We found that in simulations that include radiative cooling and SNe feedback , the SMBH grows at the Eddington limit for some periods of time presenting \langle f _ { EDD } \rangle \approx 0.5 throughout its evolution .
The \alpha parameter is dominated by the Reynolds term , \alpha _ { R } , with \alpha _ { R } \gg 1 .
The gravitational part of the \alpha parameter , \alpha _ { G } , has an increasing trend toward the galactic center at higher redshifts , with values \alpha _ { G } \sim 1 at radii \la few 10 ^ { 1 } [ pc ] contributing to the BH fueling .
In terms of torques , we also found that gravity has an increasing contribution toward the galactic center at earlier epochs with a mixed contribution above \sim 100 [ pc ] .
This complementary work between pressure gradients and gravitational potential gradients allows an efficient mass transport on the disc with average mass accretion rates of the order \sim few 1 [ M _ { \odot } / yr ] .
These level of SMBH accretion rates found in our cosmological simulations are needed in all models of SMBH growth that attempt to explain the formation of redshift 6 - 7 quasars .